Baths for the deposit of gold alloys by electroplating



United States Patent No Drawing. Application tltriyil, 1952,

Serial No. 300,127

Claims priority, application SwitzerlandMay 8,1952 6 Claims. (Cl.204-43) Baths for electrogilding, irrespective of the .gold alloy colourwhich they serve to obtain, have almost identical compositions. Indeed,;gold,.copper, silver, nickel, cadmium or zinc are, contained therein assoluble cyanides, that is to say alkaline metal-cyanides. in additionthese baths contain, according to case,"phosphates,sulphites,

carbonates and free 'cyanid'es, ith'ese jsalts being intended forimproving either the electric conductivity or the penetrating power ofthebath, or the brilliance of the deposits obtained.

When such a bath has just been prepared, it is found that, for a giventemperature and current density, its yield (considered from the point ofview of the metal deposited, as compared to the current consumed),through being lower than 100%, is the highest found in the abovementioned operating conditions. Indeed Whether the bath is used or not,it is found that the yield compared to the current consumed regularlyfalls with time and may go down to nearly 10% after a certain number ofdays: .the bath is said to have aged.

The consequences of such ageing are disastrous: when thick gilding isinvolved. (over 1 or 2 microns), the necessary duration for obtainingsuch gilding considerably increases as time passes and finally becomesunacceptable; if light gilding is involved (colour setting), the ageingstate of the bath is one more variable which must be reckoned with inthe already ticklish bath working in order to keep constant the colourof the deposited alloy.

The present invention relates to a bath for the deposition of goldalloys by electroplating, which is not subject to ageing, that is tosay, the yield of which, computed on the basis of the current consumedas compared to the weight of the metal deposited, measured when the bathhas just been prepared, remains constant during the whole life of thebath.

This bath, as those already known, contains the gold intended forplating in the form of alkaline aurocyanide. It is characterized by thefact that, apart from the said aurocyanide, it is free from any othercyanide and by the fact that it contains an organo metallic compound ofthe metal to be alloyed with gold, soluble in water and hav: ing a lowdissociation constant.

It is this organo-metallic compound, other than a cyanide, whichsupplies the metal to be alloyed with the gold at the time of formationof the gold alloy deposit. The metal of this compound may thus becopper, nickel, cadmium, zinc. It has been found that when the metal tobe alloyed with gold. is introduced into the bath in a form other thancyanide, the bath does not age.

The said organo-rnetallic compound may include in its molecule the group-N(CH2COO--)2. It may be a salt of an organic acid and of the metal tobe alloyed with gold, insofar as this salt has a low dissociationconstant. Among the salts fulfilling this condition, one may. mentionthose of the acids presenting in their molecule the above mentionedgroup, such as those of imido-diacetic acid and its derivatives, forinstance ethylene-diaminotetraacetic acid, nitrilo-tri'a'cetic acid,:anthranilic-diacetic acid, uranil-diacetic acid, aminomalonic-diaceticacid.

On the other hand, conducting s'al'ts such as phosphates and sulphitescan be completely omitted from the bath and replaced by the potassiumand sodium salts of the organo-n1etallic compound above referred to. Thebath does not contain any free cyanide.

Baths in accordance with the present invention and the organo-metalliccompound of which is a salt of ethylene-diamino-tetraacetic acid, areremarkable for their following properties:

They do not age, thatis to say that their yield,.computed on the basisof the current consumed under given electrolysis conditions, remainsconstant during their whole life;

They give brilliant gold alloy deposits, the assay of which may beadjusted by changing the electrolysis factors; They make it possible toobtain a very quick increase of the thickness of the deposits (ten timesmore quickly than with the baths usedhitherto). A current density of 2to 3 amperes per sq. decimeter (18.5 to 28 amperes .per sq. foot) at thecathode gives excellent deposits;

Their penetrating power into the cavities of the object placed at thecathode far exceeds that of ordinary cyanide baths;

They remain extremeiy limpid during use;

Their content of copper salt (being the one most generally present inthese baths) may be adjusted very easily by colorimetry owing to theblue tint of said salt.

The following examples give the composition of baths, in conformity withthe present invention, and the manner of using them:

EXAMPLE 1 Bath for the deposition of thick coatings (pink colour) g. perlitre Gold (as potassium aurocyanide) a 12 Cu (as copper salt ofethylene-diamino-tetraacetic acid) 2.5 Zn (as Zinc salt ofethylene-diamino-tetraacetic acid) 2 Ni (as nickel salt ofethylene-diamino-tetraacetic acid) 2 Potassium salt ofethylene-diamino-tetraacetic acid- 5 EXAMPLE 2 Bath for the depositionof thin gold coatings (yellow and pale yellow colours) g. per litre Gold(as potassium aurocyanide) 1 Ni (as nickel salt ofethylene-diamino-tetraacetic acid) 2 Potassium salt ofethylene-diamino-tetraacetic acid 5 By using this bath at a temperatureof 60 C. with a stainless steel anode, one may, when adjusting thecathode current density between 0.3 and 0.8 amp. per sq. decimeter (2.8and7.5 amp. per sq. foot), obtain any gold tint, from the 24 caratsyellow to very pale yellow, and all intermediate tints iii-between. Thisbath has an absolutely remarkable penetrating power and stability;

I maintaining the colour of the deposit from one operation to another isvery easy. i

By using this bath at a temperature of 60 C. with a stainless steelanode, one may, by adjusting the cathode current density between 0.3 and0.5 amp. per sq. decimeter (2.8 and 4.65 amp. per sq. foot), obtain allthe tints of green colour gold.

What we claim is:

1. A bath for the deposition of an alloy of gold with at least one metalselected from the group consisting of copper, nickel, cadmium and zinc,said bath containing exclusively an alkaline aurocyanide and at leastone organometallic compound selected from the group consisting of thecopper, nickel, cadmium and zinc salts of ethylene-diamino-tetraaceticacid, nitrilo-triacetic acid, anthranilic-diacetic acid, uranil-diaceticacid and aminomalonic-diacetic acid, wehreby the yield, computed on 4-the basis of the current consumed as compared tothe Weight of metaldeposited, remains constant during the whole life of the bath.

2. A bath according to claim 1, in which the said organo-metalliccompound is a salt of ethylene-diaminotetraacetic acid.

3. A bath according to claim 1, in which the said organo-metalliccompound is a salt of nitrilo-triacetic acid.

4. A bath according to claim 1, in which the said organo-metalliccompound is a salt of anthranilic-diacetic acid.

5. A bath according to claim 1, in which the said organo-metalliccompound is a salt'of uranil-diacetic acid.

6. A bath according to claim 1, in which the said organo-metalliccompound is a salt of aminomalonicdiacetic acid.

OTHER REFERENCES Thews: Metal Finishing, September 1951, page 83.

1. A BATH FOR THE DEPOSITION OF AN ALLOY OF GOLD WITH AT LEAST ONE METALSELECTED FROM THE GROUP CONSISTING OF COPPER, NICKEL, CADMIUM AND ZINC,SAID BATH CONTAINING EXCLUSIVELY AN ALKALINE AUROCYANIDE AND AT LEASTONE ORGANOMETALLIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF THECOPPER, NICKEL, CADMIUM AND ZINC SALTS OF ETHYLENE-DIAMINO-TETRAACETICACID, NITRILO-TRIACETIC ACID, ANTHRANILIC-DIACETIC ACID, URANIL-DIACETICACID AND AMINOMALONIC-DIACETIC ACID, WEHREBY THE YIELD, COMPUTED ON THEBASIS OF THE CURRENT CONSUMED AS COMPARED TO THE WEIGHT OF METALDEPOSITED, REMAINS CONSTANT DURING THE WHOLE LIFE OF THE BATH.